End space radiation inhibiting means for theta magnetrons



Dec. 24, 1968 E. J. COOK 3,418,521

END SPACE RADIATION INHIBITING MEANS FOR 0 MAGNETRONS Filed Dec. 27, 1965 q FIG] 4 w VI (104- w2 155% w W4 W2 3W4 n' (3 F- r" l4 0 I Q FIG.4 T Q? PRIOR ART MEANDERED 8 PORTION F I 6.3 V

I2 l3 l6 RADIAL TRANSMISSION 5 PATH 0o I'll I H 00: *I\ N M 9 Y I WH I 17 I E? Al 8 INVENTOR. l+++ RADIAL TRANSMISSION PATH BY EDWARD J. coon M 5%,; [ATTORNEY United States Patent 3,418,521 END SPACE RADIATION INHIBITING MEANS FOR 6 MAGNETRONS Edward J. Cook, South Hamilton, Mass., assignor to Varian Associates, Palo Alto, Calif, a corporation of California Filed Dec. 27, 1965, Ser. No. 516,471 8 Claims. (Cl. 31539.63)

ABSTRACT OF THE DISCLOSURE A voltage tunable magnetron is disclosed. Magnetron includes a centrally diposed cold cathode sole electrode surrounded by an interdigital anode circuit coupled to a surrounding cavity resonator. A filamentary emitter is disposed at one end of the tube. An injector electrode structure surrounds the filamentary emitter for forming and projecting an annular stream of electrons into the magnetron interaction region between the anode and the cathode sole electrodes. The injector, anode circuit, and cold cathode electrodes are axially spaced apart in insulative relation to permit independent potentials to be applied to the electrodes. The axial gaps between the injector electrode and the anode circuit, atone end, and the cold cathode electrode in the anode circuit at the other end, form a pair of radial transmission lines communicating between the magnetron interaction region and the outside of the tube. Axially coextensive structure portions are formed in the adjacent electrode structures at opposite ends of the anode circuit to define meandered porions of the radial transmission lines. These axially coextensive regions are provided at a radius substantially less than the radius which would define the low frequency cut-off for the TM 1, 0 mode in the radial transmission lines at the signal frequency to elongate that portion of the radial transmission line which is operating below cut-off for the signal energy to inhibit radiation of wave energy through the transmission lines.

Heretofore, voltage tunable magnetrons have been built which included radial gaps in the structure at one or both axial ends of the magnetron interaction region. These gaps opened into the magnetron interaction region of the tube and became coupled to the signal wave on the interaction circuit. These annular gaps served as sections of radial transmission line for radiating power out of the tube within its operating band of frequencies. In fact, it was common for the radiated power level to approach the output power level of the tube. Aside from creating the obvious problem of unwanted radio interference, the radiated power level was discontinuous over the band of the tube, producing unwanted dips in the power output spectrum of the tube. In addition, the energy storage in the radial gaps of the tube produce unwanted heating of certain of the tubes elements.

In the present invention these radial gaps are meandered back and forth to increase their length within the tube at a radius which is below their cutoff frequency for the signal wave energy. In this manner they cease to function as transmission lines for radiating power from the tube and are substantially decoupled from the signal Wave energy. As a result, the power outputspectrum is made more uniform and elimination of unwanted radio interference is made much easier.

The principal object of the present invention is to provide improved magnetron tubes having substantially reduced end space radiation of signal energy.

One feature of the present invention is the provision of a meandering path in the end space radial transmission line portions of a magnetron tube structure with the Patented Dec. 24, 1968 ice meandering portion being preferably provided at the smallest possible radius, whereby that portion of length of the radial line which is below cutoff at the signal frequency is substantially increased to inhibit radiation.

Another feature of the present invention is the same as the preceding wherein the periodic magnetron interaction circuit is of a type supporting substantial axially directed currents at the signal frequency such that end space radiation would otherwise be a specially severe problem.

Another feature of the present invention is the same as any one or more of the preceding features wherein meandering of the radial line sections is provided at both axial ends of the periodic circuit.

Another feature of the present invention is the same as any one or more of the preceding features wherein the magnetron tube is a voltage tunable magnetron having a cavity resonator coupled to and surrounding the periodic circuit and operating in the TM, 1, 0 mode at the signal frequency.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

FIG. 1 is a transverse schematic diagram of a magnetron;

FIG. 2 is the W5 diagram of a magnetron as shown in FIG. 1 utilizing a backward fundamental wave mode on the anode circuit and operating on the (N/2--l) mode.

FIG. 3 is a longitudinal sectional view of a magnetron incorporating the features of the present invention and similar to the view taken along lines 3-3 in the direction of the arrows of FIG. 1,

FIG. 4 is a fragmentary sectional view of a prior art structure similar to that portion of the structure of FIG. 3 delineated by line 4-4, and

FIG. 5 is a schematic transverse sectional view of a magnetron similar to that shown in FIG. 3 depicting the TM 1, 0 mode in the cavity of the magnetron.

Referring now to FIG. 1, there is shown in schematic diagram form a magnetron tube. The magnetron includes an anode 1 surrounding a cathode electrode 2 and defining an annular magnetron interaction region 3 between the cathode 2 and the anode 1. The anode 1 has a suitable magnetron interaction circuit 4 provided therein on the inside circumference thereof adjacent the cathode electrode 2 for cumulative electronic interaction with a stream of electrons provided in the interaction region 3 in the presence of an axially directed magnetic field B.

The anode circuit 4 as shown in FIG. 1 is merely schematic and is representative only of a circuit having a backward fundamental mode of wave propagation as depicted in the dispersion curve 6 of FIG. 2. When the magnetron interaction circuit 4 is made re-entrant, i.e., a circuit which closes upon itself, the dispersion curve for the circuit becomes discontinuous and operation can only be obtained with the circuit for electronic phase shift per period of the circuit corresponding to an integral number of full electronic wavelengths taken around the circuit.

Thus, for a circuit having N periodic elements there will be N/2 possible modes of operation as indicated by the heavy vertical lines on the dispersion curve 6. Assuming there are 8 periodic elements in the periodic anode circuit 4, there will be 4 possible modes of operation. One of the modes corresponds to the 1r mode and the closest mode to that is the (N/21) mode as designated on the dispersion curve 6. The (N/2-l) mode has be- ?come known in the art as the first 0 vmode and in a preferred embodiment of the present invention is the mode selected for operation. The first 0 has the advantage that a circuit which is constructed for operation on this mode has larger physical dimensions than a typical magnetron constructed for operation on the 1r mode at the same frequency. Thus, the first mode circuit will have greater power handling capability and is more easily coupled to for extracting output power than a similar tube constructed for 71' mode operation.

Referring now to FIG. 3, there is shown a voltage tunable magnetron incorporating features of the present invention. More specifically, the magnetron comprises a cylindrical cold cathode electrode 8 axially disposed of the tube and surrounded by an anode circuit 9 which, in a preferred embodiment of the present invention, comprises an interdigital line 10 supported at the inner periphery of a folded radial cavity resonator 11 dimensioned to have a dominant fundamental mode of resonance at the 11' mode frequency w substantially below the first 0 mode frequency w A filamentary emitter 12 is disposed at one end of the tube and is surrounded by an annular injector electrode 13 which is operated positive with respect to the emitter 12 for injecting a stream of electrons, in the presence of an axially direc-ted magnetic field B, into the magnetic interatcion region 14 defined between the anode circuit 10 and the cold cathode electrode 8.

The vacuum envelope of the tube is completed by brazing together the afore-described elements in axially spaced apart relation by means of a plurality of dielectric rings 15, as of ceramic. The ends of the vacuum envelope are completed by the outwardly flared portion of the cold cathode electrode 8 and a ceramic end cap 16.

A coaxial output line 18 is coupled to the fields of the resonator 11 via the intermediary of a cylindrical R.F. window 19 which surrounds the center conductor of the coaxial line, which center conductor is connected across the cavity 11 to form an output coupling loop for heavily coupling the magnetron circuit to a load, not shown.

In operation wave energy traveling on the magnetron interaction circuit 10 interacts with the electron stream in the magnetron interaction region 14 to produce cumulative interaction and an output signal which may be varied over a substantially wide range from 40 to an; by varying the potential applied between the col-d cathode electrode 8 and the magnetron interaction circuit 10. A tunable bandwidth of the circuit is determined by the loaded Q thereof, the higher the loading, and thus the lower the Q, the wider the band of frequencies over which the tube may be tuned. In a typical example of a tube operating at X band in the first 0 mode, the tube was tunable between 3 db points over a frequency range from 8.5 to 9.6 gc. with the tube delivering approximately watts to the load and the circuit having a loaded Q of approximately 10.

Referring now to FIG. 4, there is shown a portion of the prior art magnetron interaction structure and vacuum envelope. In the prior art structure it was found that for a tube having a power output of 5 watts, 1 to 2 watts of power was being radiated out of the tube through the insulator rings via the passages 21 which communicated with the magnetron interaction region 14. More specifically, the annular gaps in the tube structure, formed between the walls of the cavity 11 and the adjacent injector electrode 13, at one end and the adjacent cold cathode electrode 8, at the other end, provided a pair of radial transmission lines 21 which were very effectively coupled to the magnetron interaction circuit 10 and magnetron interaction region 14. Although radial passages 21 were below cutoff for the signal wave energy on the circuit 10 over short portions of the length near their inner peripheries, such cutoff lengths were not of sufficient length to prevent radiation and, as aforementioned, a very substantial fraction of the output power such as, for example, more than 50% was radiated from the tube out through the passageways 21.

The operating mode of wave propagation that exists on the anode interaction circuit 10 for the first 0 mode corresponds to a wave which has one full electrical wavelength taken about the circumference of the circuit 10. The mode on the circuit 10 also conforms to the TM 1 0 mode of resonance of the cavity 11. This TM 1, 0 mode of the cavity 11 is coupled into the passageways 21 at the axial ends of the interaction circuit 10 which opens into these passageways 21. Thus, the TM 1' 0 mode on the composite interaction circuit excites the TM 1, 0 mode in the passageways 21 and they serve as sections of radial transmission lines for this mode.

Referring now to FIG. 3, the annular passageways 21 formed by the spaces between the axially separated injector electrode 13 and the upper wall of the cavity 11 and at the bottom of the tube between the cold cathode electrode 8 and the lower wall of the cavity 11 have been substantially lengthened to lengths 1 and 1 by meandering the passageways 21 back and forth axially of the tube to substantially lengthen the path lengths 1 andl This meandering is provided in the region of smallest radius from the center line of the tube. The cutoff wavelength for these sections of radial transmission line, at the signal frequency, is about the mean inside radius of the folded cavity 11. Thus, it is readily seen that the meandering sections of the passageways 21 occur at radii substantially less than the cutoif wavelength and therefore energy will be rapidly attenuated in the meandered cutoff sections of the radial transmission lines 21. In fact, it has been found that this meandering serves to reduce the radiation of power by approximately 30 db.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a voltage tunable magnetron tube, means forming a cathode electrode structure, means forming an anode electrode structure having a periodic anode circuit portion coaxially surrounding said cathode electrode structure and defining an annular magnetron interaction region in the space between said anode circuit portion and said cathode electrode, means forming a vacuum enelope enveloping the magnetron interaction region, means for producing a stream of electrons in said interaction region for cumulative interaction with wave energy on said anode circuit portion to produce an output signal, arcuate electrode structure insulatively and axially spaced apart from and disposed adjacent one axial end of said periodic circuit portion of said anode structure to define in the axial space therebetween a radial wave passageway opening into said magnetron interaction region and communicating externally of said vacuum envelope, said axially spaced electrode structure including a concentric axially coextensive portion to define a meandered portion of said radial wave passageway, said meandered portion of said radial wave passageway being disposed at a radius from the axis of the magnetron interaction region which is substantially less than the radius which would define the low frequency cut-off for the TM 1, 0 mode in said radial wave passageway at the signal frequency, thereby elongated that portion of said radial wave passageway which is operating below cut-off for the signal energy, whereby transmission of signal 'wave energy through said radial wave passageway from said interaction region is inhibited.

2. The apparatus according to claim 1 wherein said axially spaced electrode structure which defines said radial wave passageway is provided at both axial ends of said periodic anode circuit portion, and wherein said radial wave passageways are meandered at both ends of said periodic circuit.

3. The apparatus according to claim 9 wherein said periodic anode circuit portion includes circumferentially spaced axially directed conductor portions for supporting axially directed currents adjacent said magnetron interaction region in operation.

4. The apparatus according to claim 3 wherein said anode periodic circuit portion is an interdigital line.

5. The apparatus according to claim 9 wherein said meandered portion of said radial wave passageway is disposed immediately adjacent the innermost end of said radial wave passageway to obtain maximum attenuation of radiated energy.

6. The apparatus according to claim 9 wherein said radial wave passageway defining electrode structures comprises mutually opposed surface portions of said anode electrode and of an injector electrode which is axially spaced from said anode electrode.

7. The apparatus according to claim 9 including a cavity resonator surrounding said periodic anode circuit portion and being coupled thereto, said cavity being dimensioned to support the TM 1, 0 mode at the signal frequency of said tube.

8. The apparatus according to claim 7 wherein said periodic anode circuit portion is dimensioned for operation on the first 0 mode at the output signal frequency.

References Cited UNITED STATES PATENTS HERMAN K. SAALBACH, Primary Examiner.

SAXFIELD CHATMON, JR., Assistant Examiner.

US. Cl. X.R. 

